They also work with victims and their families, and try to help others learn from fatalities, detective Doug Gold said.

They see the pieces come together.

Detectives share a wry sense of humor and a grim determination.

They work around each other, used to each other’s rhythms.

Each case has a lead, but at a crash scene, everyone pitches in.

Their pagers go off whenever there’s a serious wreck in county jurisdiction. They don’t know the details until they get there. Cars collide in infinite ways.

“Each and every case is a little different,” Gold said.

Crashes are inherently messy. Injured people need aid. There may be bodies, debris.

Detectives must document everything as best they can, taking hundreds of photographs. Cars must be towed, evidence bagged.

They’ll crawl around in the mud, looking for clues.

“Then it’s sit down for weeks or months, depending on what the case is, and what other cases you have, and systematically put that together,” Gold said.

To start, they need a map.

• • •

It’s called “total station.”

The detectives use engineering technology to create a 3-D model of the crash scene.

They use a piece of equipment that’s part of the Trimble 5600 series, the yellow tripod you see on the side of the road when someone is surveying, or police are investigating a recent crash.

The machine uses lasers, prisms and computer software to create map-points for key spots and pieces of evidence.

In the old days, detectives measured scenes with footsteps or paces, Gold said. Then, they got tape measures.

In the 1990s, total station technology became affordable and accessible, and law enforcement jumped on. The tools are more accurate than anything used before, Gold said.

Where things land can reveal the path they followed, detective Joe Goffin said.

It’s physics: Action produces reaction. Impact sends a spray.

The detectives do their best to determine the speed that vehicles were moving. Sometimes it’s not possible. Witnesses might be among the dead. People might not talk, or are injured so severely they may never remember.

The detectives gather everything they can. They try to put the crash back together.

“You get raw data to start with,” detective George Metcalf said.

• • •

The detectives revisit the scenes, over and over.

They need similar weather conditions to test the road. They estimate how far each driver could have seen ahead. They bring out similar cars, and recreate the view from behind the wheel, based on each driver’s height.

“Things come up a little different from everyone’s angle,” Gold said.

Everything is photographed, measured and sometimes videotaped.

They use spray paint to mark the pavement, a shorthand in neon squiggles and dots.

At one scene, on a cold morning this spring, they gathered on a wooded country road near Arlington.

Passing drivers did their customary brake-tap as they spotted the squad cars lining the road.

Vehicles driven in hit-and-runs can have strands of the victim’s hair stuck in the glass. Paint transfers between objects scraped during impact. People leave behind genetic material: blood, skin, bone.

“This is one of the few units that forensically does 90 percent of its own work,” Fenter said.

Crashes also cause injuries that detectives analyze to evaluate survivors’ stories, like the different marks seat belt straps leave, depending on where somebody was sitting.

It all goes into the file, layer upon layer.

• • •

Collision investigations take longer than most. It might be six months or a year before a prosecutor decides whether to file charges.

All crimes must be reconstructed, Snohomish County deputy prosecutor Paul Stern said. The sheriff’s C.I.U. detectives must explain each case so prosecutors, the defense and a jury can understand.

“They’re using the law of physics and their training to tell us what happened to the vehicles,” deputy prosecutor Tobin Darrow said.

The detectives’ focus on details, and their willingness to admit their own margins of error, makes them good witnesses on the stand, Stern said.

“There are limited factual disputes when you’re dealing with mathematical and scientific formulas,” he said.

The detectives aim to investigate each case so thoroughly that guilty people won’t risk trial. That saves a family from testimony, and also saves public expense.

• • •

The last piece of a case can become its conclusion.

That’s what the detectives are looking for, after all: an answer.

Why was he hurt? Why did she die?

“You just let the evidence lead you where you need to go,” Metcalf said.

Rikki King: 425-339-3449; rking@heraldnet.com

Traffic safety classes

Snohomish County sheriff’s detective Doug Gold teaches free traffic-safety programs for parents and teens. The “Driving It Home” program includes presentations by people who have lost someone to impaired driving and displays of actual cars ripped apart in crashes.